1. Technical Field
The present invention relates to a liquid crystal display device, and in particular, to a method of driving a liquid crystal display device.
2. Related Art
Some display devices use cathode-ray tubes (CRTs). Other display devices may be flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays (FED), and electro-luminescence displays (ELDs). Some of these flat panel displays may be driven by an active matrix driving method in which a plurality of pixels arranged in a matrix configuration are driven using a plurality of thin film transistors. Among these active matrix type flat panel displays, liquid crystal display (LCD) devices and electroluminescent display (ELD) devices may exhibits a higher resolution, and increased ability to display colors and moving images as compared to some of the other flat panel display devices.
A LCD device may include two substrates that are spaced apart and face each other with a layer of liquid crystal molecules interposed between the two substrates. The two substrates may include electrodes that face each other. A voltage applied between the electrodes may induce an electric field across the layer of liquid crystal molecules. The alignment of the liquid crystal molecules may be changed based on an intensity of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device may display images by varying the intensity of the electric field across the layer of liquid crystal molecules.
FIG. 1 is a block diagram of a LCD device according to the related art, and FIG. 2 is a circuit diagram of a liquid crystal panel of FIG. 1.
Referring to FIGS. 1 and 2, the LCD device includes a liquid crystal panel 2 and a driving circuit 26. The driving circuit 26 may include gate and data drivers 20 and 18, a timing controller 12, a gamma reference voltage generator 16, an interface 10 and a power generator 14.
Referring to FIG. 2, the liquid crystal panel 2 includes a plurality of pixels. The plurality of pixels are connected to a plurality of gate lines GL1 to GLn along a first direction and a plurality of data lines DL1 to DLm along a second direction. Each pixel includes a thin film transistor TFT and a liquid crystal capacitor LC. The liquid crystal capacitor LC includes a pixel electrode connected to the thin film transistor TFT, a common electrode, and a liquid crystal layer between the pixel and common electrodes. The common electrode is supplied with a common voltage.
The interface 10 is supplied with data signals and control signals such as a vertical synchronization signal, a horizontal synchronization signal, a data enable signal, and a data clock signal. The data signals and control signals are supplied from an external system, such as a computer system.
The timing controller 12 is supplied with the control signals from the interface 10 and generates control signals to control the gate and data drivers 20 and 18. The timing controller 12 processes data signals and supplies those to the data driver 18. The gate driver 20 is supplied with the control signals from the timing controller 12 to sequentially output gate voltages to the gate lines GL1 to GLn. The gate lines GL1 to GLn are sequentially enabled, and the thin film transistors TFT connected to the enabled gate line GL1 to GLn are turned on. The data driver 18 is supplied with the data signals and the control signals from the timing controller 12. The data driver 18 outputs data voltages to the data lines DL1 to DLm when the gate line GL1 to GLn is enabled. A gamma reference voltage generator 16 generates gamma reference voltages which are supplied to the data driver 18. The power generator 14 supplies voltages that operate the components of the LCD device.
An inversion method may be used to operate the LCD device. In the inversion method, the data voltages alternately have opposite polarities every predetermined pixel and every predetermined frame. Accordingly, deterioration of liquid crystal molecules is prevented.
For the LCD device operated in the inversion method, when a static image is scrolled, an after-image may occur along a moving path. This is referred to as a scroll after-image.
FIG. 3 is a view illustrating a scroll after-image in the LCD device according to the related art.
Referring to FIG. 3, when a static image shown with a solid line moves right to left in a liquid crystal screen 50, a scroll after-image shown with a dashed line appears along a moving path according to the scroll. This problem is caused by a DC voltage accumulation in the pixels along the moving path. In other words, the pixel on the moving path is repeatedly supplied with data voltages having the same polarity, and thus a DC voltage of such the polarity is accumulated in the pixel.
FIG. 4 is a table of data voltage polarities causing a DC voltage accumulation when a static image is scrolled with a predetermined scroll pattern in an LCD device according to the related art.
Referring to FIG. 4, a static image is white, and a background of the static image is gray. The predetermined is that when the static image is scrolled, the static image moves in a speed of N pixel/frame, for example, 8 pixel/frame and a white data voltage is inputted to a pixel, which is located on a moving path, every M frames according to the speed, for example, 8 frames.
In the related art, each pixel has positive and negative polarities alternately every frame according to a one-dot inversion method. Each pixel has positive and negative polarities alternately every two frames according to a first two-dot inversion method and a second two-dot inversion method.
Accordingly, when the scroll operation is conducted with the above inversion methods, the white data voltages having the same polarity, for example, a negative (−) polarity continue to be inputted to the pixel every 8 frames. In other words, this input of the white data voltages having the same polarity with a specific number of frames occurs commonly in the one-dot inversion method and the first and second two-dot inversion methods. Accordingly, a DC component of the same polarity is gradually accumulated in the pixel as the scroll operation continues, and this causes an after-image due to the scroll operation. In particular, as the speed gets lower, the static image stays at the pixel for a longer time and input frequency of the white data voltage having the same polarity increases. Accordingly, the DC voltage accumulation increases, and thus the after-image appears more.